I attempted my first crack at brewing Mead on 7/17. I added 14 lbs honey to 5 gal water to bring OG to 1.090. I put in bucket and covered, but did not make air tight. I saw no fermentation for 1 week.

On 7/24 I aerated thoroughly and added yeast nutrient.

Waited 1 week and took gravity reading on 7/31 - it read 1.100. Is that possible? I also tasted it, and it tasted as though it was fermenting, but my readings are saying something different. I added more yeast (White Labs Champagne) and with some yeast nutrient.

On 8/5 it seemed as though it was fermenting slowly.

Here are my questions:

Should I see more signs of fermentation at this point?

Is there a point where the must is beyond kick starting into fermentation?

my first thought is that you measured your SG wrong somewhere. while temp can affect an SG reading 10 points is WAY out of the realm of possibility. 14 pounds of honey in 5 gallons of water should of gotten you a higher OG than 1.090. you may also want to check the PH of the must. if its below about 3.7 or so the yeast may be unhappy and not wanting to work for you. you can try warming the must up a little to give the yeast more energy to work with.

As stated before, you most likely had layering of honey with more sitting lower and a lighter mix above that your measured.
Hightest has a spreadsheet in the sticky at the top of this forum that will take you from a guess, to straight math as long as you get you volumes right.
Personally I have read his FAQ a number of times and each time new light bulbs go off that make me improve my process.
Otherwise break out the hydrometer, and read up on SNA, and then just make a good mead.

As far as temp, my house is air conditioned with a constant temp of 75 degrees.

Having never made mead before, I still am not sure if the must can go bad. Is there a point of no return where I need to toss the must and start over? I am thinking no, but would hate to go through the entire process only to find out that I wasted my time.